This invention relates to a tool for mounting disk valves in motor cylinder heads, the disk valves being of the type that on its rod has a number of transversal, circumferential grooves for accomodating a corresponding number of beads on the insides of two substantially semi-circularly shaped locking bodies, which taper conically from the rear ends towards the front ends and which have the purpose of securing a locking ring against the valve spring, comprising a shaft, which in the region of one end has on one hand a spring-loaded tap, that is pressable into an outer cavity in the end portion of the shaft, and on the other hand two cross-sectionally vaulted clamping arms that are placed on opposed sides of the shaft, which clamping arms are articulatedly connected with the shaft and activated by at least one spring means that always strives to keep the arms swivelled inwards against the shaft, and against the action of which the clamping arms are pivotable outwards to a position distal from the shaft and the tap, the clamping arms in the region of the free ends having means for keeping said locking bodies in a given axial position relative to the clamping arms, in order to make possible a transfer of the locking bodies to the valve rod, more precisely by positioning the tap against the rod and pressing the same into the cavity of the shaft.